Metal tank

ABSTRACT

The invention discloses the manufacture of a metal tank comprising the following steps: coating a strip of sheet steel with a thermally adhesive polymer based layer, preparing segments provided with assembly zones from the blanks of the said coated sheet, assembling the said segments at the assembly zones, consolidating the assembly by cold mechanical assembling means and heating to activate bonding of the polymer layers in contact at the assembly zones. The invention is useful in particular for manufacturing fuel tanks and heat exchangers.

The invention concerns the making of metal tanks intended to contain atleast one fluid and capable of resisting pressure, even of gas, as forexample automobile fuel tanks, heating radiators for buildings orrefrigerator evaporators or condensers.

To make such tanks from sheet metal blanks, said blanks are put into theform of shells having a shape adapted to the tank to be made, then saidshells are assembled to form the tank.

The assembly is generally achieved by welding in the region ofassembling zones or flanges in order to ensure both liquid-tightness andresistance to pressure.

In the case where the tank is intended to contain corrosive liquids, inparticular in the case of fuel tanks, steel sheets are currently usedwhich are coated with a coating for protection against internalcorrosion.

The U.S. Pat. No. 4,824,736 (exactly corresponding to WO-89 02855)describes in particular a sheet steel tank coated internally with aprotective coating of polypropylene modified with maleic anhydride.

The protective polymer coating maintains a very strong adherence to thesheet during and after the shaping of the shells.

But the presence on the sheet of a protective polymer coating may betroublesome for the shell welding operation: indeed, the thermal effectsof the welding often produce a deterioration of the polymer coating inthe vicinity of the shell-welding bead, and the internal zone of theweld bead is no longer effectively protected against corrosion.

In the case where the sheet is provided with a coating providingprotection against external corrosion, the deterioration of the externalcoating, also consecutive to the welding operation, requires "repairing"the protective coating in the welding zone, for example by applicationof an external protective coating in this zone after welding.

To avoid the drawback relating to the nature of the sheet protectingcoatings, one could be tempted to effect the welding of the shells underconditions adapted to limit the heating of the polymer coating to asimple melting.

Apart from the drawback of requiring a very strict control of thewelding conditions, such an assembling process still leaves the weldingbead itself without protection against corrosion.

The document U.S. Pat. No. 4,131,980 describes a process for making ametal tank, in which:

shells are put into shape from metal sheets which are not provided witha polymer coating,

the assembling flange of one of the two metal shells to be assembled isprovided with a polymer coating (reference 36 in FIG. 2 of thedocument),

the two flanges are seamed one around the other (FIGS. 3 to 7 of thedocument).

The polymer coating applied then acts as an adhesive joint between theflanges folded for seaming.

The two assembling means, the mechanical seaming means and the chemicalbonding means, collaborate.

As concerns the bonding, the use of epoxy vinyl, polyester,polyurethane, polysulphide resin is described.

This process has several drawbacks:

the use of this type of resin as the bonding means poses problems ofinternal corrosion in that the adhesive joint is liable to bedeteriorated by the fuel

this process requires an operation for applying glue on the assemblingflanges after the shells have been put into shape;

with reference to FIG. 3B which represents, in section, the junctionbetween two shells 4' and 5' with the glue joint 6', it was found thatthe zone termed the triple point zone designated by the reference 7B,which corresponds to the limit of the adhesive joint 6' in contact withthe two sheets 4' and 5', formed a mechanically weakening line or arupture initiation line and that it is in this region that the risk ofrupture of the tank was the highest.

The invention also concerns the making of metal heat exchanger tanksintended in particular for making evaporators and condensers ofrefrigerating machines.

The following description concerns more particularly evaporators of the"roll bond" (in the English language) type, but the same process may beemployed for making any heat exchanger (for example: heating radiator orsolar panel, condenser) employing one or more cooling or heat transferfluids.

To make evaporators of the "roll bond" (in the English language) type:

usually two sheets of aluminium alloy are taken,

one of the sheets is locally provided with a coating of ananti-diffusion product on a zone outlining the circuit intended for thecooling fluid in the evaporator,

the second sheet is applied on the partly-coated surface of the firstsheet and the two assembled sheets are hot rolled to produce locally awelding by diffusion between these two sheets, outside the zone providedwith the anti-diffusion product between these two sheets,

a fluid under pressure is then introduced in the non-welded zone betweenthe two assembled sheets so as to form ribs by expansion of the sheetsin this zone.

There is thus obtained an evaporator tank panel formed of two stronglyassembled ribbed sheets which are therefore fluid-tight and resist thepressure of gas, the ribs of which form the cooling fluid circuit in theevaporator.

The process termed "roll bond" (in the English language) permits makingexchangers in the form of a panel having a large thermal exchangesurface; these "flat" exchangers are also easy to clean, which presentsadvantages from the hygienic point of view.

Such a process is, for example, described in the document EP 703 427.

This process has several drawbacks:

the operations for welding and shaping by application of a fluid underpressure are costly.

it requires the use of a material which is weldable by diffusion and issufficiently ductile to be easily formable under pressure of a fluid;this is why aluminium is generally used, which is an expensive material.

it imposes constraints as concerns the definition of the fluid circuits,the forming by blowing being liable to produce ruptures in particular inthe curved parts of the circuit.

An object of the invention is to provide an economical tank formed of aplurality of assembled metal shells which have a better resistance topressure, are protected internally against corrosion by a polymercoating, including in the region of the shell assembling zones.

The invention is applicable in particular to the making of fuel tanksand heat exchangers, in particular evaporators and condensers ofrefrigerating machines or heating radiators.

For this purpose, the invention provides a process for making a sheetsteel tank intended to contain a fluid, resisting pressure of a gas,characterized in that it comprises the steps consisting in:

coating a strip of said sheet with a thermobonding polymer based coatingadapted to adhere to said sheet and resist corrosion of said fluid,

then cutting out blanks in said coated strip and putting into shapeshells in accordance with the shape required for said tank, providedwith assembling zones, in particular assembling flanges, intended to beput into contact upon assembly, said shells being prepared in such amanner that the coated face of the sheet is situated inside the tank,

then assembling said shells in such a manner as to put them in contactunder pressure in the region of the assembling zones, reinforcing theassembly by cold mechanical assembling means applied in the region ofsaid assembling zones, such as seaming or clinching, and heating saidpolymer coatings in contact in the region of the assembling zones, at atemperature adapted to activate the bonding and form a tight bondedjoint.

The invention may also have one or more of the followingcharacteristics:

said polymer based coating has a thickness of between 20 and 150micrometers.

said sheet of steel is coated with a metallic coating, in particular ofzinc, aluminium, zinc alloy, or aluminium alloy and/or a coating ofpaint.

The invention also provides a process for making a tank intended tocontain automobile fuel, characterized in that:

the polymer of said coating is semi-crystalline thermoplastic,

the heating of said polymer coatings in contact in the region of theassembling zones is effected at a temperature higher than the meltingtemperature of said polymer.

The invention may also have one or more of the followingcharacteristics:

said semi-crystalline thermoplastic polymer is based on polyolefine, inparticular polyethylene or polypropylene

said semi-crystalline thermoplastic polymer has a crystallinity ratiohigher than 60% and a solubility parameter close to that of said fuel.

The invention also provides a process for making a tank intended to actas a heat exchanger and to contain a cooling or heat-transfer fluid,characterized in that the shaping of the shells in the shape requiredfor said heat-exchanger comprises the formation of ribs adapted toconstitute after assembly, tight ducts for the circulation of said fluidin said exchanger.

The invention finally provides a tank intended to act as aheat-exchanger, characterized in that it comprises a plurality ofindependent thermal fluid circulation circuits.

The invention will be better understood from a reading of the followingdescription, which is given as a non-limitative example, with referenceto the accompanying Figures, in which:

FIG. 1 represents, viewed from above, an evaporator tank panel accordingto the invention and FIG. 2 represents in section along AA' a part ofthe evaporator of FIG. 1.

FIGS. 3A and 3B are diagrammatic sectional representations of theadhesive joint between two tank shells, respectively according to theinvention and according to the prior art.

FIG. 4 represents two shell-assembling means employed in the processaccording to the invention, on one hand, both the seaming on anassembling flange and the clinching in an assembling zone, on the otherhand, the adhesive joint in the region of the flange and assemblingzone.

FIG. 5 represents partly in section an evaporator of the same type asthat of FIG. 1, but having two independent cooling fluid circuits.

The installation for making a tank comprises means for coating a stripof steel with a polymer coating, means for cutting out sheet blanks fromthe sheet strip, means for shaping the sheet blanks, means for the coldmechanical assembling, as for example, seaming or clinching means, andmeans for heating to a temperature adapted to activate the bonding ofthe polymer coatings in contact.

These means are known per se and will not be described here in detail.

The process for making the metal tank from a sheet strip will now bedescribed.

The sheet strip may be already precoated with a metallic coating forprotection against corrosion, such as a coating of zinc, aluminium, zincor aluminium alloys.

This prior coating is in particular useful for protecting the exteriorsurface of the tank against corrosion.

Again, as concerns the exterior of the tank, this steel strip may evenbe precoated with a coating of paint (termed "prelacquered" steel) onthe face intended to subsequently form the external surface of the tank.

With the aid of coating means, there is applied, preferablycontinuously, on the face of the steel strip intended to formsubsequently the internal surface of the tank, a thermobonding polymerbased coating.

This thermobonding polymer may be for example a semi-crystallinethermoplastic polymer.

This thermobonding polymer may be a thermosetting composition thecross-linking of which is thermally activatable.

This thermobonding polymer based coating may contain colouring agents,pigments, plasticizers and other additives conventional in the field ofvarnishes or paints.

The method of application is known per se and will not be described herein detail; it may involve the use of solvents or a drying or evaporationoperation,

To ensure a good adherence of the polymer coating on the sheet, beforeapplication, the surface of the sheet is preferably treated by forexample degreasing it and applying a chemical conversion solution suchas a chromating or phosphatizing solution,

Preferably, the thickness of the polymer coating applied is between 20and 150 μμm so as to be able to form, after the bonding opperationsubsequently described, a continuous adhesive joint which is thereforetight and strong.

To impart to the steel sheet good resistance to corrosion by the fluidof the tank, the applied polymer must be both insoluble or very littlesoluble in this fluid and stongly adhere to the steel sheet.

To ensure a good adherence of the polymer coating to the sheet, there ispreferably chosen a polymer modified by grafting or copolymerizationwith polar compounds, preferably selected from acrylic, vinyl, acetate,maleic anhydride and acrylic esters.

In the case of automobile fuel tanks, there is used as the polymer, asemi-crystalline thermoplastic polymer, preferably a polyolefine, inparticular a polyethylene or polypropylene.

To limit the sensitivity of the polymer to fuels, it is consequentlypreferable that the applied polymer have a crystallinity ratio higherthan 60% and a solubility parameter close to that of the fuel to becontained (for example about 8 (cal/cm³)⁰.5 for petrol).

In the case of a heat exchanger tank, there may usually be usedthermoplastic polymers, such as a mixture of polyethylene and polyamide,or thermosetting compositions such as prepolymers of epoxy resin whichare simple or modified to improve the tenacity.

To avoid any degradation or cracking when shaping or even assembling, apolymer having a vitreous transition temperature lower than about 10° C.is preferably chosen.

After coating the sheet strip with a coating of polymer, it is arrangedto make the tank in several segments or shells.

For example, for a fuel tank having complex shapes, the tank issegmented into shells which have a press-formable shape and haveassembling flanges applicable against one another for assembling thetank.

When the tank to be made is a heating radiator, conventionally, it isalso arranged to make the radiator in a plurality of segments to beassembled; these segments also have assembling zones which areapplicable against one another, in particular in the region of theflanges.

When the tank to be made is a heat exchanger 1 of a refrigeratingmachine (evaporator or condenser) such as that shown in FIG. 1 having aduct 3 for the circulation of the cooling fluid, it is arranged to makethe exchanger from two elements, here sheets 4,5 ribbed for example by apress-forming operation; the ribs 2 of these sheets are adapted to formsaid duct and the planar non-ribbed surfaces of the sheets are adaptedto form the assembling zones of the to sheets 4,5.

There are therefore provided on the shells or elements of the tank, onthe face coated with polymer, assembling zones, in particular on theflanges, applicable against one another when assembling.

The area of these zones is adapted to the chosen cold mecanicalassembling mode; for example for the seaming of the flanges, rather wideflanges should be provided which will be subsequently folded.

The area of these zones is also adapted to the stiffness and theresistance to pressure required of the tank.

In the case of heat exchangers such as that shown in FIG. 1, theposition of the assembling zones defines the thermal fluid circuit orcircuits and the large area of the assembling zones permits obtaining avery high resistance to pressure, for example up to 40 10⁵ Pa.

With the aid of cutting means, the sheet blanks are then cut out in thestrip coated with polymer; then, in starting with these blanks and withthe aid of shaping means, the tank shells or elements are prepared inthe manner known per se, which permits, among other things, locating theassembling zones and, in the case of heat exchangers, the thermal fluidcircuit or circuits.

For these operations, care is taken to maintain the polymer basedcoating on the side forming the internal surface of the tank.

Depending on the shape of the tank shells or elements, it is possible toemploy shaping methods other than press forming, in particular folding.

The different shells or elements are then assembled to constitute thetank by applying the assembling zones against one another and, in thecase of assembling flanges, flange against flange.

In the region of the interface between the assembling zones of theadjacent shells or elements, there are therefore two polymer basedcoatings which come into contact on the surface of the assembling zone.

With the aid of cold mechanical assembling means, the shells or elementsare interconnected, for example by seaming or clinching in theassembling zones and/or flanges.

Then, while the shells or elements are already partly interconnected andmaintained by the contact pressure in these zones, the sheet is heatedwith the aid of heating means in these assembling zones so as toactivate the bonding (or thermobonding) of the two coatings of polymerin contact in these zones.

The heating means may for example employ infrared radiation, magneticinduction or conduction.

If a thermoplastic polymer is used, the heating means are adapted tobring the surface of the contacting polymer coatings to a temperature,on one hand higher than the melting temperature of the polymer and onthe other hand distinctly lower than the degradation temperature of thepolymer or other prior coating or prior lacquering of the sheet.

A tank which is fluid-tight and very resistant to pressure is obtained.

The polymer coating then performs the double function of internalprotection against corrosion by the contained fluids and a tight andstrong joint between the shells or elements forming the tank.

As the polymer coating may be continuously applied on the strips, it canbe distributed in an improved manner over the sheet to better ensurethis double function.

According to the invention, the inner surface of the tank remainstotally protected against corrosion, even in the region of theassembling zones.

As the polymer coating is continuous inside the tank, there is no longerany mechanically weakening line 7B as in the document U.S. Pat. No.4,131,980 previously cited with reference to FIG. 3B; indeed, thejunction zone 7A of the shells or elements of the tank are then as shownin FIG. 3A, with no rupture-initiating line and the tank according tothe invention resists higher pressures.

Owing to the interconnection by the cold mechanical assembling meansprior to the heating for achieving the bond, the adhesive joint isformed under perfectly static conditions, which improves its resistance.

Owing to the high adherence of the polymer to the sheet, the adhesivejoint effectively contributes to the interconnection of the shells.

The bonding and the mechanical assembling in the assembling zonesconsequently affords a very strong joint between the shells and a veryhigh stiffness of the tank.

Depending on the desired resistance, there is adapted in the mannerknown per se the type of clinching or the number of clinching points inthe assembling zones. The tank is tight in the region of the assemblingflanges in particular owing to the adhesive joint formed by the polymerbased coating and resists pressure owing to the very strong jointbetween the shells.

In the case of heat exchangers subjected to very high pressures, forexample in the case of refigerator evaporators or condensers, theadhesive joints in the assembling zones provide a tight insolationbetween the elements of the thermal fluid circuit or circuits and afforda high resistance to pressure.

Such a "mixed" assembling process (bonding+cold mechanical assembling)allows high production rates since the part can be handled during andabove all immediately after the bonding heating.

Further, the bonding is here achieved with no substance added to thesheet after shaping (no application of adhesive), which considerablysimplifies the production (no handling of products, no risks related tosolvents).

In the case of heat exchangers, in particular refrigerators, the processaccording to the invention is much more economical than the processes ofthe prior art, in particular the process termed "roll bond" in theEnglish language.

The definition of the fluid circuits by forming ribs prior to theassembly permits, contrary to the process employing fluid expansionapplied after assembly, forming with precision complex circuits with norisk of rupture.

Another advantage of this process is that is permits economically makingheat exchangers having a plurality of independent thermal fluidcirculation circuits.

In the case of for example fuel tanks, another advantage of this processis that it is easy to obtain tanks having flanges which are muchnarrower than in the case of assembling by welding, while retaining acomparable resistance to pressure; for example, the flanges of tanksaccording to the invention would extend 5 to 10 mm outwardly beyond thebody of the tank in the case of seaming, about 10 to 20 mm in the caseof clinching, but more than 20 mm for the welded tanks of the prior art.

Thus, for a given capacity, the outside dimensions may be reduced (theflanges projecting much less than in the case of welded tanks), which isparticularly advantageous in the case of automobile tanks.

Another advantage of this process is to permit the assembly of shellswhose flanges define contours of small radius of curvature (contrary towelding), which permits making tanks of complex shape.

A metal tank was thus made for fluids which is capable of resistingpressure and completely protected against internal corrosion by apolymer based coating including in the region of the assembling joints.

Further, in the case where the sheet used is precoated with a metalliccoating or prelacquered, for reasons of external appearance of the tankor resistance to external corrosion, the precoating or the prelacqueringis not deteriorated by the tank-making operations.

Without departing from the present invention, the application of thepolymer coating on the sheet is effected continuously on an industrialsheet-production site and the products delivered from this site formaking tanks are then strips of sheet which are "precoated withadhesive" on one face and may also be prelacquered on the other face.

Advantageously, the manufacturer of the tanks has no welding to effectnor any adhesive or solvent to handle.

The following examples illustrate the invention.

EXAMPLE 1

The purpose of this example is to illustrate the first step of making atank according to invention, namely the preparation of a strip of sheetprepainted on one face and coated on the other face with an adhesivepolymer film, by means of a continuous lacquering installation and arolling installation.

The sheet used is of galvanized steel (about 150 g/m² of zinc basedprecoating) about 0.7 mm thick.

The polymer film of the coating is based on an ethylene-acrylic acid("EAA") copolymer having 11% acrylic acid; the melt index measured inaccordance with the ASTM D 1238 standard is 8 g per 10 min; its densitymeasured in accordance with the ASTM D 1505 standard is 940 kg/m³ ; itsmelting temperature measured in accordance with the ASTM E 794 standardis 94° C.

This coating film is made by extrusion-blowing and has a thickness of100 μm.

To make the prepainted and coated strip of sheet, the sheet is degreasedin an alkaline bath, a chromating treatment is applied which comprises achromic rinsing, a paint is applied on the face to be painted in thelacquering installation and then the polymer coating film is rolled ontothe other face by means of the rolling installation at the approximaterate of 60 m/min and at a temperature of about 200° C.

The tensile-shear and peeling tests then carried out on the film appliedon the sheet reveal a very good adherence of the film to the sheet, evenafter 7 days storage under damp conditions.

In the case of the application of the same film on the same sheet on apress under 0.2 MPa and at 120° C. The following adherence strengths areobtained:

for the tensile-shear, the strength exceeds 11 MPa.

for the peeling, the strength exceeds 6 daN/cm.

In both cases, after unsticking, there is obtained a profile termed"surface cohesive rupture", which indicates that the rupture occurswithin the thickness of the film of adhesive applied and not in theregion of the film-sheet interface.

EXAMPLE 2

The purpose of this example is to illustrate the resistance to pressurethat can be obtained for tanks made in accordance with the invention.

With a painted and coated sheet according to example 1, a tank of aboutfive liters capacity is made.

To make this tank, the shells are put into shape and assembled accordingto the invention, the cold mechanical assembling means employed beingseaming.

The seal tests carried out show that this tank resists a pressure ofabout 7 10⁵ Pa and that the resistance to pressure is limited only bythe deformation of the metal shells which form this tank.

This value of the resistance to pressure is distinctly higher than theusual specifications which are generally of the order of 0.4 10⁵ Pa.

EXAMPLE 3

The purpose of this example is to illustrate the use of the processaccording to the invention for making a heat exchanger panel 1 such asthat shown in FIG. 1.

A strip of steel sheet is continuously coated with a thermobondingpolymer coating.

Two sheets 4,5 are cut out in this strip which has been precoated withadhesive.

The size of the sheet 1 is slightly larger than that of the sheet 5 soas to permit subsequently the seaming of the flanges of the sheet 4 ontothose of the sheet 5.

The two sheets 4,5 are press-formed in such a manner as to form ribs 2which are hollow on the coated face.

The two sheets 4,5 are applied against one another, the two coated facesbeing in contact, so that the ribs of the sheet 4 form with the ribs ofthe sheet 5 thermal fluid circulation ducts 3.

The flanges of the sheet 4 are seamed onto those of the sheet 5 asindicated by reference 8 of FIG. 4 and clinchings 9 are formed atpredetermined points of the assembling zones of the tank.

The number and the position of the clinchings are predetermined inaccordance with the desired resistance to pressure.

While maintaining the two sheets 4,5 pressed against each other, theassembly is heated to activate the bonding between the two polymercoatings in the region where they are in contact, i.e. in the assemblingzones.

An adhesive joint 6 which is tight and strong is in this way formed onthe whole of the area of the assembling zones.

A heat exchanger panel 1 is in this way obtained.

In respect of ribs having a width of 5 mm, it has been possible, owingto a thermobonding polymer having a rupture energy measured in shearhigher than 3 kJ/m², to ensure a resistance to pressure of the exchangerexceeding 4 10⁶ Pa.

EXAMPLE 4

The purpose of this example is to illustrate the use of the processaccording to the invention for making a heat exchanger panel of the sametype as that shown in FIG. 1 but having two independent thermal fluidcirculation circuits 3A, 3B.

The procedure is as in example 3 except that:

a strip of sheet steel precoated on both faces is also prepared,

a sheet 10 of the same size as the sheet 5 is cut out in this strip,

the sheet 10 is interposed between the sheet 4 and the sheet 5 uponassembly.

There is then very economically obtained a heat exchanger panel such asthat shown in section in FIG. 5 having two independent thermal fluidcirculation circuits 3A,3B.

We claim:
 1. A process for producing a tank of sheet steel intended tocontain a fluid, resisting pressure of a gas, comprising:coating a stripof said sheet with a thermobonding polymer based coating adapted toadhere to said sheet and resist corrosion of said fluid, then cuttingout blanks in said coated strip and producing shells which are shaped inaccordance with the shape intended for said tank and include assemblingzones, for putting in contact when assembling, said shells beingprepared in such a manner that the coated face of said sheet is situatedinside said tank, then assembling said shells in such a manner as to putthem in contact under pressure in the region of said assembling zones,reinforcing the assembly by cold mechanical assembling means applied inthe region of said assembling zones, and heating said polymer coatingsin contact in the region of said assembling zones to a temperatureadapted to activate the bonding and form a fluid-tight bonding joint. 2.The process of claim 1, wherein said cold mechanical assembling means isa seaming.
 3. The process of claim 1, wherein said cold mechanicalassembling means is clinching.
 4. The process of claim 1, wherein saidpolymer base coating has a thickness of between 20 and 150 micrometers.5. The process of claim 1, wherein said steel sheet is coated with acoating selected from the group consisting of zinc, aluminium, zincalloy, aluminium alloy, paint.
 6. The process of claim 1, wherein saidsteel sheet is coated with a coating selected from the group consistingof zinc, aluminium, zinc alloy, aluminium alloy and with a coating ofpaint.
 7. The process of claim 1, said tank being intended to contain anautomobile fuel, wherein:said polymer of said coating issemi-crystalline thermoplastic, said heating of said polymer coatings incontact in the region of said assembling zones is effected at atemperature higher than the melting temperature of said polymer.
 8. Theprocess of claim 7, wherein said semi-crystalline thermoplastic polymeris based on polyolefin.
 9. The process of claim 8, wherein saidpolyolefin is polyethylene.
 10. The process of claim 8, wherein saidpolyolefin is polypropylene.
 11. The process of claim 7, wherein saidsemi-crystalline thermoplastic polymer has a crystallinity ratio higherthan 60% and a solubility parameter close to that of said fuel.
 12. Theprocess of claim 8, wherein said semi-crystalline thermoplastic polymerhas a crystallinity ratio higher than 60% and a solubility parameterclose to that of said fuel.
 13. The process of claim 1, said tank beingintended to act as a heat exchanger and contain a cooling orheat-transfer fluid, wherein said shaping of said shells in accordancewith the shape intended for said heat exchanger comprises forming ribsadapted to constitute, after assembly, fluid-tight ducts for thecirculation of said fluid in said exchanger.
 14. A metal fuel tankformed of a plurality of shells of steel sheet assembled in afluid-tight manner, obtained by a process comprising:coating a strip ofsaid sheet with a thermobonding polymer based coating adapted to adhereto said sheet and resist corrosion by said fuel, then cutting out blanksin said coated strip and producing shells which have a shape inaccordance with the shape intended for said tank and includingassembling zones, for putting in contact when assembling, said shellsbeing prepared in such a manner that the coated face of said sheet issituated inside said tank, then assembling said shells in such a manneras to put them in contact under pressure in the region of saidassembling zones, reinforcing the assembly by cold mechanical assemblingmeans applied in the region of said assembling zones, and heating saidpolymer coatings in contact in the region of said assembling zones to atemperature adapted to activate the bonding and form a fluid-tightbonding joint, said shells being internally coated with a coating ofsemi-crystalline thermoplastic polymer for protection against internalcorrosion, and said polymer coating acting as a fluid-tight adhesivebonding joint between said assembled shells.
 15. A tank intended to actas a heat exchanger and contain a cooling or heat-transfer fluid, formedof a plurality of shells of steel sheet assembled in a fluid-tightmanner obtained by a process comprising:coating a strip of said sheetwith a thermobonding polymer based coating adapted to adhere to saidsheet and resist corrosion of said fluid, then cutting out blanks insaid coated strip and producing shells which are shaped in accordancewith the shape intended for said heat exchanger tank and includeassembling zones, for putting into contact when assembling, said shellsbeing prepared in such a manner that the coated face of the sheet issituated inside said tank, then assembling said shells in such a manneras to put them in contact under pressure in the region of saidassembling zones, reinforcing the assembly by cold mechanical assemblingmeans applied in the region of said assembling zones, and heating saidpolymer coatings in contact in the region of said assembling zones to atemperature adapted to activate the bonding and form a fluid-tightbonding joint, said shape of said shells in accordance with the shapeintended for said heat exchanger comprising ribs which constitute in theassembled shells fluid-tight ducts for the circulation of said fluid insaid exchanger.
 16. The tank of claim 15, comprising a plurality ofindependent thermal fluid circulation circuits.